You can always buy more than you need and freeze the leftovers. It will make the henna stain much darker when thawed out. I would start with 300 grams and see how that works. I'm sure you will have some leftover.
All henna is an orangey red and it oxidizes to a darker red in a few days. The only real difference in them is the dye content. Rajasthani has a lower dye content and Yemen has a high content. Jamila is a very popular choice for many. I use Celebration from Mehandi.com and it has the highest dye content and gives me a nice ruby red. A good thing about henna is you can try all the different ones and not damage your hair.
I'm sure the Lush hennas had some indigo mixed in, so using a pure henna powder will be a different red. The more you use it, the darker red you will get.